Service tax

One our client is providing Management consultancy service, for this service he collects service tax and remits regularly. And part of his another project he has service appartment business. For the second project he has not collected service tax since he ventured it. Now  we came to know that service tax has to be collected and paid on the second venture also. My question is that should we pay arrears of service tax along with interest? and If I leave the arrears amount and start collecting service tax now on, what will be the consequence?